Mechanism And Application Of TiO₂ Nanotube-based Materials For Photocatalytic Removal Of VOCs

Volatile organic compounds（VOCs）,generated from numerous anthropogenic activities（e.g.,industrial and indoor material emissions）,would pose a range of threats to human health and environmental quality.Therefore,the efficient removal of VOCs has become an urgent and crucial issue in the field of atmospheric environment management.Photocatalytic technology can convert harmful organic pollution molecules into harmless CO₂and H₂O at room temperature and pressure,and has the advantages of low cost,simple operation,wide application range and less secondary pollution,so it is a very promising technology for VOCs removal.For the photocatalytic degradation of VOCs,the development of stable and efficient photocatalytic materials is the technical key.Among the numerous photocatalysts,TiO₂nanotubes are considered to be well suited for VOCs photocatalytic removal technology because of their high specific surface area,fast intra-tubular mass transfer,and low accumulation of carbon bed leading to deactivation.However,the large-scale application of TiO₂nanotubes is still limited by its severe photo-generated electron-hole pair complex and wide band gap.In view of this,TiO₂nanotubes were used as the research object in this thesis.And a series of TiO₂nanotubes-based composites were prepared for the gas-phase degradation of VOCs through modification methods including heterojunction design,defect engineering,single-atom engineering,and construction of gas-phase photoelectrocatalytic system.Then,the conformational relationship between the photoelectric properties of the materials and the degradation performance of VOCs was investigated.Finally,An air-cleaning air conditioner based on the optimized material was designed to explore its potential application in indoor air cleaning.The research contents and results are as follows:1.Defective WO₃/TiO₂nanotubes for photocatalytic removal of volatile organic compounds.WO₃/TiO₂nanotube heterojunctions were prepared by the anodic oxidation and electrochemical deposition,and then oxygen vacancies were introduced into the material by a Moderate electrochemical self-doping approach.The results showed that the content and distribution of OVs can be precisely regulated by controlling reduction duration and potential..The introduction of surface OVs in the material reduced carrier transfer resistance and significantly promoted carrier separation.As a result,the photocurrent density of-1.4 V sample prepared at 600 s duration was increased by a factor of 12,and performance of photocatalytic removal of toluene compared with the pristine material nanotubes was greatly improved.2.Au single atoms anchored WO₃/TiO₂nanotubes for photocatalytic removal of volatile organic vompounds.A simple two-step electrochemical approach was developed to synthesis a atomically dispersed Au loaded WO₃/TiO₂nanotubes array for VOCs oxidation.Au atoms was proved to be anchored by OVs on WO₃surface,thus exhibiting a1.8 m A/cm²photocurrent,17 times than that of the pristine materials,and the highest efficiency in degrading the toluene.The enhancement in performance was attributed to the significantly improved carrier transport and separation as well as toluene adsorption.More importantly,the strong metal-support interaction led to the thermodynamic stability of the Au single atoms,and thereby the stable toluene degradation cycle was achieved.Furthermore,according to the mechanistic analysis,the carrier transfer of Au single atoms-WO₃/TiO₂nanotubes followed a“Z-Scheme”and that·OH and·O₂^-radicals were the main active species.In addition,according to the the intermediate product analysis,the toluene mineralization followed the following pathways:benzyl alcohol-benzaldehyde-benzoic acid-benzoquinone-small molecules such as formic acid,acetic acid and maleic acid-CO₂and H₂O.3.Gas-phase photoelectrocatalytic oxidation of volatile organic compounds using defective WO₃/TiO₂nanotubes mesh.A defective WO₃/TiO₂nanotubes array mesh was developed and used as photoelectrode in a ducted reactor equipped with 365 nm LED light source to purify VOCs gas.Introduction of defects into the photoelectrode by an electrochemical reduction step significantly improved its carrier concentration and transfer performance.By applying direct small bias potential to the photoelectrodes,the photo-electrons were separated rapidly from the holes.More importantly,due to the quick transfer of photo-electrons,the recombination of the mobile electrons with the holes trapped by bulk OVs was significantly reduced,thereupon more photogenerated holes and electrons were set free to generate·OH and·O₂^-radicals.Thus,applying bias potential to the defective photoelectrode results in almost 3.5 times higher kinetic rate for toluene degradation.The ducted reactor was found to be shown better stability and higher performance in the purification of VOCs.4.Research on air-cleaning air conditioner based on defective WO₃/TiO₂nanotubes.The performance comparison,economic calculation and life cycle assessment of the above three VOCs degradation systems were carried out,and the best VOCs degradation system,i.e.,a gas-phase photocatalytic system based on defective WO₃/TiO₂nanotube films,was preferentially selected.Then,the defective WO₃/TiO₂nanotube fins were prepared on a large scale and integrated with 365 nm fluorescent tubes into the evaporator of a indoor air conditioner hanging unit to design an photocatalytic/photoelectrocatalytic air-cleaning conditioner for indoor formaldehyde removal.According to the results of formaldehyde decay experiments conducted in a homemade 3 m³experimental chamber（with reference to GB/T 18801-2015）,the clean air delivery rate（CADR）of the air-cleaning air conditioner in photocatalytic and photoelectrocatalytic modes were 1.134 and 2.484 m³/h,respectively,and their the energy efficiency ratings were 0.07 and 0.13 m³/（W·h）,respectively.This design gives a non-adsorption air-cleaning function without changing the original structure of the air conditioner,which has great practical significance and commercial prospect.